This invention relates to a pneumatic actuator adapted to expand its diameter to cause a contraction force in axial directions by introducing a pressurized fluid thereinto. In particular, it relates to a pneumatic actuator capable of detecting relative movements between ends of the pneumatic actuator to control their positions when the pneumatic actuator is contracted.
Such a pneumatic actuator adapted to contract in axial directions while expanding in the radial direction by application of a pressurized fluid has a lot of advantages in that it is very light weight and easy to control owing to its smooth movement in comparison with actuators using electric motors or hydraulic cylinders. For example, a pneumatic actuator as shown in FIG. 1 has been known from Japanese Patent Application No. 40,378/77. The pneumatic actuator shown in FIG. 1 comprises a tubular body 1, a reinforcing braided structure 2 arranged externally thereon, closure members 3 at both ends and clamp sleeves 4.
The tubular body 1 is preferably made of a rubber or rubber-like elastomer which is air-impermeable and flexible. However, other materials equivalent thereto, for example, various kinds of plastics may be used for this purpose.
The reinforcing braided structure 2 is made of cords whose braided angles are approximate to what is called an angle of repose (54.degree.44') when the tubular body 1 is expanded at the maximum with the pressurized fluid supplied thereinto. The cords are organic or inorganic high tensile strength fibers, preferably, for example, twisted or nontwisted filament bundles of aromatic polyamide fibers (trade name, KEVLAR) or very fine metal wires.
One of the closure members 3 is formed at least on one side with a connecting aperture 8 communicating with an inner cavity 7 of the tubular body 1 through an aperture 6 formed in the nipple 5 in its axial direction. A fitting 9 is fitted in the connecting aperture 11 of the closure member 3. To the fitting 9 is connected an operating pressure source, for example, an air compressor (not shown) by a line including a flow control valve. With this arrangement, when a controlled pressure is applied into the inner cavity 7 of the tubular body 1, the braided angles of the reinforcing structure 2 are enlarged to cause "pantagraph movement" of the reinforcing cords of the braided structure 2, so that the diameter of the tubular body 1 is expanded and the axial length thereof is contracted resulting therefrom to shorten a distance between pin apertures of the closure members 3.
With such a pneumatic actuator adapted to displace in its axial direction with the controlled pressure applied thereinto, however, the tubular body made of a rubber or rubber-like elastic material and the braided structure exhibit so-called "hysteresis error" when they expand or contract. As the result, their contracted lengths are different when the pressurized fluid is being supplied into and exhausted from the inner cavity of the tubular body. In order to determine their contracted lengths exactly, therefore, it is required to adjust the pressure of the pressurized fluid taking account of the hysteresis characteristics of the tubular body and the braided structure. It may unavoidably lower its operating efficiency.
With the pneumatic actuator above described, moreover, its contractive force cannot be directly determined, and due to the hysteresis characteristics it is required to calibrate the relation between the pressurized fluid to be applied and the contractive force. If the pneumatic actuator is used in a driving means which is required to know the contractive force caused by the pneumatic actuator exactly, detecting means is additionally needed for detecting the contractive force. Accordingly, the merit of the air bag type pneumatic actuator which is of light weight and inexpensive is spoiled and the space to be occupied by the pneumatic actuator increases.
In the above pneumatic actuator, moreover, pressure detecting means for detecting the pressure in the pneumatic actuator is provided in a line between the pneumatic actuator and an operating pressure source. Accordingly, there are various problems such as leakage of the pressurized fluid in the pressure detecting means and the line and limitation of location where the pneumatic actuator is arranged. Moreover, an operator does not know the pressure of the pressurized fluid serving to expand the actuator because the pressure cannot be directly detected.